Microwave spectrum of dimethylallene excited states and strong Coriolis coupling

The rotational spectra of six excited vibrational states of dimethylallene were measured and assigned to the corresponding vibrational levels, and for three more excited state spectra at least the rotational constants could be determined. Between the two lowest excited levels of symmetry species b₂ and b₁ of group C_2v a strong a-type Coriolis coupling was found to exist. The evaluation of the resulting perturbation by a diagonalization of the energy matrix yielded ζ^(a) = 0.36 and a precise value for the vibrational energy difference 48.761 GHz (1.6 cm^?1). The state b₂ is believed to be the first excited torsional substate (01, 10)₁ of methyl internal rotation, and the rotational transitions of this state as well as those of the strongly coupled state b₁ presented very irregular multiplet splittings. On the other hand, the splittings of the next-higher excited state of species a₂ which could be identified as the partner torsional substate (01, 10)₂, followed the regular pattern, yielding an internal rotation barrier V₃ (2079 cal/mole) not unlike that derived earlier from ground state splittings.     

Microwave spectrum of axial cyclohexyl chloride and conformational equilibrium in cyclohexyl chloride

The high-resolution microwave spectrum of the axial conformer of cyclohexyl chloride has been assigned for both the Cl(35) and Cl(37) isotopic species, in natural abundance. Moreover, the high-resolution microwave spectrum of the equatorial conformer, previously assigned in the ground state [Chem. Phys. Lett.21, 592–594 (1973)], has been further investigated. Several low-energy vibrational satellites have been studied for the Cl(35) isotopic species of both the axial and the equatorial isomers. The ground state centrifugal distortion constants for both the isotopic species of the two rotamers, and the quadrupole coupling constants of Cl(35) and Cl(37) axial isomer and of Cl(37) equatorial isomer (not reported previously) have been determined. From line relative intensity measurements the energy difference between the two conformers has been calculated to be 0.51 ± 0.15 kcal/mol, the equatorial isomer being more stable. In the same way, the vibrational energies of the investigated vibrational satellites have been computed. A structural model for axial cyclohexyl chloride is proposed on the basis of the available experimental data.     

Internal rotation and inversion doubling in the microwave spectrum of CH3NHCl

The microwave “a” and “c” type spectra of four isotopic species of CH₃NHCl in the ground state and of CH₃NHCl³⁵ and CH₃NDCl³⁵ in the first excited torsional state have been analyzed. From the A-E torsional splittings of the excited state the torsional barrier height has been determined to be V₃ = 3710 ± 46 cal/mole. The “c” type transitions show an inversion doubling of 4.60 ± 0.10 MHz in the ground state and of 5.25 ± 0.10 MHz in the first excited torsional state. Such doublings are independent on the rotational quantum numbers within the experimental errors. The height of the inversion barrier has been roughly evaluated by using the Dennison-Uhlenbeck potential.     

Microwave spectrum and barrier to internal rotation of 1-chloro-2-butyne

Rotational transitions of the μ_a and μ_b type have been identified with microwave-microwave double resonance measurements for 1-chloro-2-butyne in the ground vibrational state. In the first excited state of the methyl torsion only μ_a-type transitions have been identified. The A-type transitions of the ground vibrational state can be described perfectly by the rigid rotor approximation with centrifugal corrections. Using the internal axis method the barrier to internal rotation was determined from the A,E splittings: V₃ = 10.05 ± 0.09 cm⁻¹. A model which allowed for geometry relaxation upon internal rotation was used to fit one set of parameters to the transition frequencies of both ground state and first excited torsional state. The sixfold contribution to the barrier was found to be negligible: V₆ = −0.4 ± 0.3 cm⁻¹.     

Microwave spectrum of 2-chloropyridine

The microwave spectrum of 2-chloropyridine, C₅H₄NCl, has been studied in the frequency range from 26.5–40.0 GHz. The spectrum is characterized by strong parallel type transitions of a near-prolate asymmetric top. The assigned transitions have been used to evaluate the ground state rotational constants of the two chlorine isotopes. The rotational constants are (in MHz): A = 5872.52, B = 1637.83, C = 1280.48 for the ³⁵Cl isotopic species and A = 5872.16, B = 1591.76, C = 1252.17 for the ³⁷Cl isotopic species. The small inertial defect indicates the molecule is planar. In addition an excited vibrational state of C₅H₄N³⁵Cl has been observed and analyzed. The chlorine quadrupolar coupling constants were determined for the ground state and are: χ_aa = ?71.9 MHz for ³⁵Cl and χ_aa = ?54.9 MHz for ³⁷Cl. By assuming the pyridine ring structure the CCl bond length is found to be 1.72 Å.     

The microwave spectrum of isotopically substituted hypochlorous acid: Determination of the molecular structure

Microwave spectra have been measured for four isotopically substituted species of hypochlorous acid (D¹⁶O³⁵Cl, D¹⁶O³⁷Cl, H¹⁸O³⁵Cl, H¹⁸O³⁷Cl). Both a- and b-type transitions have been analyzed for rotational, centrifugal distortion, and Cl nuclear quadrupole coupling constants. The distortion constants, together with vibrational wavenumbers, have been used to evaluate a valence harmonic force field. Effective, substitution, ground state average, and estimated equilibrium structures are presented.     

The 14N quadrupole coupling constants and barrier to internal rotation of ethylcyanide-d5

The microwave spectrum of ethylcyanide-d₅ has been recorded from 18.0 to 40.0 GHz. Both a-type and b-type transitions were observed and assigned. Also, the R-branch assignments have been made for three excited states of the internal torsional mode and two excited states of the CN inplane bending mode as well as an excited vibrational state involving both of these motions. The barrier to internal rotation was determined to be 3.00 ± 0.15 kcal/mole from the E, A splittings of the third excited state. The quadrupole coupling constants of the¹⁴N nucleus were found to have values of ?3.213, 1.168, and 2.045 MHz for χ_aa, χ_bb, and χ_cc, respectively. These results are compared to those previously obtained on the corresponding hydrogen compound.     

The microwave spectrum of CHD2Cl

The ground vibrational state microwave spectrum of CHD₂Cl has been studied in the region 26.5–40.0 GHz. From the observation of weak c-type transitions the A₀ rotational constants of CHD₂³⁵Cl and CHD₂³⁷Cl have been determined to be 95 426.08 ± 0.06 and 95 425.23 ± 0.11 MHz, respectively. The observed a-type and c-type transitions have been used to obtain A, B, C, all five quartic and one sextic distortion constants present in the reduced Hamiltonian of Watson for the ³⁵Cl and ³⁷Cl isotopic modifications of CHD₂Cl.     

Microwave spectra of eight isotopic modifications of 1-chloro-1-fluoroethylene

The rotational spectra of eight isotopomers of 1-chloro-1-fluoroethylene in the 6-22 GHz region have been collected and analyzed. Each rotational transition is split into hyperfine components by the chlorine (either ³⁵Cl or ³⁷Cl) nuclear quadrupole coupling interaction and additionally, one or more smaller interactions such as the spin-rotation interaction due to the fluorine atom, hydrogen-hydrogen spin-spin coupling interactions, and in appropriately substituted species, the deuterium nuclear quadrupole hyperfine interaction. The rotational constants derived from these isotopomers allow the determination of average and Kraitchman substitution structures for 1-chloro-1-fluoroethylene, whereas the availability of the diagonal chlorine nuclear quadrupole coupling constants for all the isotopomers provides complete quadrupole coupling tensors for both ³⁵Cl and ³⁷Cl. In the course of this work, the rotational spectrum of an excited vibrational state of the normal isotopomer was observed, which ab initio calculations suggest should be assigned to ν₉=1, an in-plane bending motion at the CFCl end of the molecule.     

Microwave spectrum,quadrupole coupling constant,and conformation of 1-cyanocyclohexene

The microwave spectrum of 1-cyanocyclohexene has been investigated in the frequency regions 8–12.4 and 18–26.5 GHz. A-Type transitions in the ground state and three excited states have been assigned. The rotational constants in the ground state were determined to be A = 4565.98 ± 0.46, B = 1423.44 ± 0.01, and C = 1136.17 ± 0.01 MHz. From the experimental data, it was suggested that the molecule has an equilibrium half-chair conformation similar to those of cyclohexene and 1-fluorocyclohexene. From the hyperfine splittings of the ¹⁴N nucleus, the quadrupole coupling constant, X_aa, was found to be 4.2 MHz.     

Millimeterwave rotational spectrum and internal rotation in o-chlorotoluene

The millimeterwave rotational spectrum of o-chlorotoluene is investigated in the frequency region 150-250 GHz. Many rotational lines show splitting due to internal rotation of the methyl group. The analysis of the internal rotation splitting allows us to determine with precision the potential barrier to internal rotation of the methyl group. However, it is found that the moment of inertia of the methyl top is probably much smaller than usually assumed, which significantly affects the value of the barrier. Accurate centrifugal distortion constants are obtained for the ground states of ³⁵Cl and ³⁷Cl isotopologues as well as for an excited vibrational state.     

The millimeter wave spectrum of phosphorus oxychloride

Millimeter wave rotational spectra of phosphorus oxychloride (OPCl₃) in the ground and excited vibrational states have been recorded and analyzed. The v₅ = 1 and v₆ = 1 state spectra show large splittings due to l resonance and the effect of the 2, -1 term r_t. Coriolis constants have been obtained for the two lowest degenerate states. The spectra of the asymmetric top species OP³⁵Cl₂³⁷Cl have been analyzed and centrifugal distortion constants obtained. These have been used to determine the harmonic force field of the molecule.     

Microwave spectrum,dipole moment,and structure of 3-aminopropanol

The microwave spectra of 3-aminopropanol and three of its deuterium substituted isotopic species have been investigated in the 26.5 to 40 GHz frequency region. The rotational spectrum of only one conformer has been assigned in which presumably a hydrogen bond of the OH---N type exists. The rotational spectra of a number of excited vibrational states have been observed and assignments made for some of these excited states. The average intensity ratio for the rotational transitions between the ground and excited vibrational states indicates that the first excited state is about 120 cm^?1 above the ground state.and the next higher state is roughly 200 cm^?1 above the ground vibrational state. The dipole moment was determined from the Stark effect measurements to be 3.13 ± 0.04 D with its principal axes components as |μ_a| = 2.88 ± 0.03 D, |μ_b| = 1.23 ± 0.04 D and |μ_c| = 0.06 ± 0.01 D. The possibility of another conformer where the hydrogen bond could be of NH---O type was explored, but the spectra of such a conformer could not be identified.     

Microwave spectrum of fluoroacetyl chloride

The microwave spectrum of fluoroacetyl chloride has been studied in the 8–40 GHz region and transitions arising from one conformer have been assigned. This conformer has all the heavy atoms in a plane with the fluorine and chlorine atoms trans to one another. The rotational constants and nuclear quadrupole coupling constants for the ground vibrational state are (in MHz): H₂FCCO³⁵Cl: A = 9025.82, B = 2403.92, C = 1920.70, χ_aa = ?47.7, χ_bb = 23,7, χ_cc = 24.1; H₂FCCO³⁷Cl: A = 8994.95, B = 2342.24, C = 1879.75, χ_aa = ?38.0, χ_bb = 18.9, χ_cc = 19.1. The spectrum of the first excited torsional state has been assigned. Some lines possibly due to a second conformer have been observed but not yet assigned.     

The high-resolution infrared spectrum of Si2H6: rotation–torsion analysis of the ν5 and ν7 fundamentals,and torsional splittings in the degenerate vibrational states

The infrared active ν₇ and ν₅ fundamentals of disilane, coupled by an x,y Coriolis interaction, have been analysed on a Fourier transform spectrum between 2120 and 2225?cm^?1, at the Doppler limited spectral resolution. A Fermi resonance with 2ν ₂?+?ν₉ affects the Δ K?=?1 side of ν₇, and both ν₇ and ν₅ show the effects of several additional localized perturbations. Line splittings in the ν₅ transitions are not observed, showing that the torsional splitting in the ν₅ excited state and in the vibrational ground state are almost equal. The intrinsic torsional splitting of ν₇ is found to be smaller than in the ground vibrational state by 0.0085?cm^?1. This splitting value and those found for the other two infrared active degenerate fundamentals, ν₈ and ν₉, follow the trend expected from our theoretical predictions. Exploratory numerical calculations show that the decrease of the torsional splittings, observed in the fundamental degenerate vibrational states of disilane, can actually be accounted for by the head–tail and torsional Coriolis coupling of all the degenerate vibrational fundamentals, in several torsional states.     

The microwave spectrum of methyl chlorodifluoroacetate: Methyl internal rotation and chlorine nuclear electric quadrupole coupling

A chirped pulse microwave spectrometer has been used to record microwave spectra of the ³⁵Cl and ³⁷Cl isotopologues of methyl chlorodifluoroacetate, CClF₂C(O)OCH₃, between 8 GHz and 16 GHz. The target compound was spectroscopically examined as it participated in a supersonic expansion of argon. Only one conformer was observed. The rotational spectra were recorded with sufficient resolution to observe (i) splittings due to the internal rotation of the methyl group, and (ii) splittings from the coupling of the chlorine quadrupolar nucleus. A total of 785 transitions have had quantum numbers assigned. Analysis of the spectra observed has produced an experimental barrier to the methyl group internal rotation, V₃, of 370(2) cm⁻¹. It is noted that this barrier is a little lower than that determined for methyl acetate [V₃ = 425 cm⁻¹, J. Sheridan, W. Bossert and A. Bauder, J. Mol. Spectrosc., 80 (1980) 1-11], and this is rationalized through a comparison of molecular structures. Lastly, all components of both the ³⁵Cl and ³⁷Cl chlorine nuclear electric quadrupolar coupling tensor have been determined.     

Microwave spectrum and quadrupole coupling constant tensor of chloroiodomethane

The microwave spectrum of ³⁵Cl and ³⁷Cl species of chloroiodomethane was investigated in the frequency region of 9–35 GHz. The b-type R-branch and Q-branch transitions were assigned. The rotational constants of the ground state were determined to be A = 27 418.81 ± 0.10, B = 1621.879 ± 0.024, and C = 1545.730 ± 0.044 MHz for the ³⁵Cl species; and A = 27 261.46 ± 0.16, B = 1562.240 ± 0.047, and C = 1491.008 ± 0.092 MHz for the ³⁷Cl species. From the hyperfine splitting of the I, ³⁵Cl, and ³⁷Cl nuclei, the nuclear quadrupole coupling constants were determined to be χ_aa = −1404.5 ± 3.8, χ_bb = 383.4 ± 2.1, χ_cc = 1021.1 ± 4.3, and ∥χ_ab∥ = 1176.5 ± 3.6 MHz of iodine; χ_aa = −26.8 ± 2.3, χ_bb = −11.0 ± 1.2, and χ_cc = 37.8 ± 2.6 MHz of ³⁵Cl for the ³⁵Cl species; χ_aa = −1423.2 ± 5.5, χ_bb = 389.1 ± 2.9, χ_cc = 1034.1 ± 6.2, and ∥χ_ab∥ = 1170.2 ± 6.9 MHz of iodine; and χ_aa = −20.4 ± 3.3, χ_bb = −9.1 ± 1.8, and χ_cc = 29.5 ± 3.7 MHz of ³⁷Cl for the ³⁷Cl species, respectively. The centrifugal distortion constants were also determined using all of the assigned transitions. A brief discussion of the procedure for analyzing the quadrupole hyperfine structures of a molecule containing two quadrupolar nuclei is also provided.     

Rotational spectra, nuclear quadrupole hyperfine tensors, and conformational structures of the mustard gas simulent 2-chloroethyl ethyl sulfide

Rotational spectra have been recorded for both the ³⁵Cl and ³⁷Cl isotopic forms of two structural conformations of 2-chloroethyl ethyl sulfide (CEES). The rotational constants of the ³⁵Cl and ³⁷Cl isotopomers were used to identify the conformational isomers. A total of 236 hyperfine transitions have been assigned for 47 rotational transitions of the ³⁵Cl isotope of a GGT conformer, and 146 hyperfine have been assigned for 37 rotational transitions of the ³⁷Cl isotopomer. For the second conformer, a total of 128 (110) hyperfine and 30 (28) rotational transitions have also been assigned to the ³⁵Cl (³⁷Cl) isotopes of a TGT conformation. The extensive hyperfine splitting data, measured to high resolution with a compact Fourier transform microwave spectrometer, were used to determine both the diagonal and off-diagonal elements of the ³⁵Cl and ³⁷Cl nuclear quadrupole coupling tensors in the inertial tensor principal axis system. The experimental rotational constant data, as well as the ³⁵Cl and ³⁷Cl nuclear quadrupole coupling tensors, were compared to the results from 27 optimized ab initio (HF/6-311++G^∗∗ and MP2/6-311++G^∗∗) model structures.     

Analysis of the quadrupole coupling in the millimeter-wave spectrum of trichlorosilane

The splittings due to the three chlorine quadrupoles, observed in the millimeter-wave rotational spectra of Si³⁵Cl₃H in the ground and ν₆ = 1 excited vibrational states, have been analyzed and the quadrupole parameters determined. In particular, splittings observed for the kl − 1 = −1 lines in ν₆ = 1 are explained in terms of the asymmetry parameter η_Q. The value obtained is consistent with the electric field gradient at the ³⁵Cl nuclei having cylindrical symmetry about the SiCl bond.     

Far-infrared spectrum and ground state constants of methyl amine

The far-infrared spectrum of methyl amine has been studied in the region 40 to 350 cm⁻¹ by Fourier transform spectroscopy with an apodized resolution of 0.005 cm⁻¹ or better. Both the pure rotational spectrum and the spectrum of the fundamental torsional band have been assigned. This paper reports the ground state constants obtained from a global fitting of a data set including ground state microwave transitions from the literature, as well as far-infrared pure rotational ground state transitions and ground state combination differences from the torsional band obtained in this work. Slightly over 1000 energy differences for the ground state with 0 ≦ K ≦ 19 and K ≦ J ≦ 30 were fit to 30 molecular parameters from a group theoretical formalism developed earlier, and a standard deviation of ±0.00063 cm⁻¹ was obtained. An ambiguity (noted earlier in the microwave literature) in the determination of the structural parameter ϱ, which arises when two large amplitude motions are present in the molecule, can be understood and resolved using the present formalism.